120 



ANALYSIS OF THE ENVIRONMENT 



heavier (Bergmann's rule) (Allen, 1905; 

 Ogle, 1934). Similarly, the young of the 

 common domestic fowl kept at 6° C. during 

 their third and fourth months of life were 

 shorter in body length, gained more weight, 

 and had shorter tarsi and tails than did their 

 former flock mates, which were kept 

 throughout at 21 to 24.5°. The birds from 

 the lower temperature also had larger 

 hearts, as has been reported for birds in 

 nature (Hesse, 1921; Hesse, Allee, and 

 Sclimidt, 1937; Allee and Lutherman, 

 1940). 



An interesting sidelight on the relation 

 between internal and external temperatures 

 with respect to extremities throws important 

 light on phenomena such as those that 

 doubtless underlie Allen's rule. Red bone 

 marrow, ordinarily absent from the distal 

 regions of the tail in many animals, will 

 form if the intact tail tip is inserted into and 

 retained in the warm body cavity by a sim- 

 ple surgical operation (Huggins and Block- 

 som, Jr., 1936). Conversely, spermatozoa 

 of certain animals with pendant testes, such 

 as sheep, will not develop if the tempera- 

 ture is raised to that normally found within 

 the body cavity (Moore and Quick, 1924). 



Poikilothermous terrestrial animals tend 

 to have their species and individuals with 

 largest size in warmer, rather than in colder, 

 climates. In this, a main trend in their sur- 

 face-mass geographic relations difi^ers from 

 the general rule for homoiotherms. Terres- 

 trial lizards, snakes, and many insects have 

 their larger species, or individuals within a 

 species, in the warmer parts of their range. 



Exceptions occur to both the homoiother- 

 mal and poikilothermal phases of this rule. 

 Among mammals, there are many, of which 

 racoons (Prociion) aflFord an example, in 

 which the body size becomes smaller to- 

 ward the north. The reduction in body size 

 corresponds with an invasion of a less suit- 

 able climate. Hibernating mammals and 

 migrating birds escape the full rigors of 

 winter cold and may show no relation be- 

 tween body size and environmental tem- 

 peratures. Small birds have difficulty in 

 maintaining an even, high body tempera- 

 ture in a variable climate and may be lim- 



ited to the tropics except for summer 

 migrations; the hummingbirds give an ex- 

 ample. An exception to the usual rule that 

 in terrestrial, cold-blooded forms the body 

 size is largest toward the tropics is fur- 

 nished by bumblebees, which are fuzzier 

 and larger in the northern part of their 

 range. These are evidently adjustments that 

 conserve the body heat generated by the 

 action of large wing muscles. Here we have 

 another example of the frequent experience 

 of ecologists. When different principles 

 come into conflict, only a direct inquiry can 

 determine which will be followed in any 

 given instance. It is worth repeating that 

 while we can discern and outline many 

 broad general ecological principles with 

 confidence, their application in a given 

 situation is frequently a matter for empiri- 

 cal research. 



CONCLUDING STATEMENT 



Temperature is an important factor in the 

 physical environment of organisms. Its rel- 

 ative value must not be judged by the 

 length of the present chapter as compared 

 with others in this section. Neither has the 

 available information about the temperature 

 relations of animals been summarized in 

 overgreat detail. It is easy to measure tem- 

 perature with a high degree of accuracy 

 even under field conditions, (unfortunatelv 

 without necessarilv identifying the critical 

 temperatures involved). Sturdy recording 

 thermometers are available that require 

 a minimum amount of attention. Also, tem- 

 peratures are relatively easy to control in 

 the laboratory. Finally, in addition to its 

 known general importance, man has long 

 been much interested in temperature on his 

 own account, and this subjective factor, 

 combined with ease of measurement, has 

 led to the accumulation of a vast amount of 

 seasoned information about temperature as 

 an ecological factor, not all of which has 

 been surveyed here. The interested student 

 is referred to the bibliography for work 

 done to date and to Biological Abstracts for 

 the steady flow of new data dealing with 

 temperature. 



